1. Field of the Invention
The present invention relates to a nitride-based top emission type light emitting device and a method of manufacturing the same. More particularly, the present invention relates to a nitride-based top emission type light emitting device having a large area and high capacity and a method of manufacturing the same, in which an ohmic modification layer is interposed between a p-nitride cladding layer and a transparent conducting layer, thereby improving electro-optical characteristics of the nitride-based top emission type light emitting device, such as the external quantum efficiency (“EQE”).
2. Description of the Related Art
III-nitride-based semiconductors are direct-type semiconductor materials having widest band gaps used in optical semiconductor fields. Such III-nitride-based semiconductors are used to fabricate high efficient light emitting devices capable of emitting light having wide wavelength bands in a range between a yellow band and an ultraviolet band. However, although various endeavors have performed for several years in various industrial fields to provide the light emitting device having a relatively large area, high capacity, and high brightness, such endeavors have ended in a failure due to the following basic difficulties related to materials and technologies.
First, a difficulty of providing a substrate adapted to grow a nitride-based semiconductor having a high quality.
Second, a difficulty of growing an InGaN layer and an AlGaN layer including a great amount of indium (In) or aluminum (Al).
Third, a difficulty of growing a p-nitride-based semiconductor having a relatively higher hole carrier density.
Fourth, a difficulty of forming a high-quality ohmic contact electrode (e.g., ohmic contact layer) suitable for an n-nitride-based semiconductor and a p-nitride-based semiconductor.
Nevertheless of the above difficulties derived from materials and technologies, in late 1993. Nichia chemicals (a Japanese Company) developed a blue light emitting device by using a nitride-based semiconductor. A white light emitting device included a high brightness blue/green light emitting device coupled with a phosphor. Such a white light emitting device is practically used in various illumination industrial fields.
In view of performance of a light emitting device, such as a light emitting diode (“LED”) or a laser diode (“LD”) employing a high-quality nitride-based semiconductor, characteristics of an ohmic contact formed between a semiconductor and an electrode are very important factors.
Nitride-based LEDs are classified into top-emission type LEDs and flip-chip type LEDs based on the emission direction of light generated from a nitride-based active layer. In the case of the top-emission type LED, the light generated from the nitride-based active layer is emitted to an exterior through a p-ohmic contact layer that makes contact with a p-nitride-based cladding layer. Therefore, a high-quality p-ohmic contact layer is necessary in order to obtain a nitride-based top-emission type LED having a high quality. Such a nitride-based top-emission type LED must have a higher light transmittance of 90% or more, and a non-contact ohmic resistance value as low as possible. That is, in order to fabricate a next-generation nitride-based top emission type LED having the high capacity, large area, and high brightness, superior ohmic contact characteristics are essentially necessary to simultaneously perform the current spreading in the lateral direction and the current injecting in the vertical direction such that a high surface-resistance value of a p-nitride-based cladding layer caused by a low hole density can be compensated. In addition, a transparent p-ohmic contact electrode having a higher light transmittance must be provided in order to minimize light absorption when the light generated from the nitride-based active layer is output to the exterior through the p-type ohmic contact layer.
FIG. 1 is a cross-sectional view showing a conventional nitride-based top emission type light emitting device, such as is available from Nichia Chemicals of Japan.
Referring to FIG. 1, the conventional nitride-based top emission type light emitting device includes a substrate 110. A nitride-based buffer layer 120, an n-nitride-based cladding layer 130, a multiple quantum well nitride-based active layer 140, a p-nitride-based cladding layer 150, and a p-ohmic contact layer 160 are sequentially formed on the substrate 110. Reference numerals 170 and 180 represent a p-type electrode pad and an n-type electrode pad, respectively.
The p-ohmic contact layer 160 is provided with a p-ohmic contact electrode including a thin semi-transparent nickel-gold (Ni—Au) electrode having superior ohmic contact characteristics or a transparent conducting oxide layer, such as an indium tin oxide (ITO) layer. However, although the p-ohmic contact electrode employing semi-transparent nickel-gold (Ni—Au) has superior electrical ohmic contact characteristics, it represents a low light transmittance, so that a large amount of light is absorbed in the p-ohmic contact electrode when the light generated from the multiple quantum well nitride-based active layer 140 is emitted to the exterior. Thus, the light emitting device represents a low EQE.
The ITO electrode represents superior light transmittance, so that the EQE of the light emitting device can be improved. However, the ITO electrode forms a schottky-type contact causing a relatively large voltage drop, rather than the ohmic contact, on the p-nitride-based cladding layer 150, so that the current injection can not be readily achieved. Thus, if the ITO electrode is used as the p-ohmic contact electrode, the nitride-based light emitting device having high capacity, large area and high brightness cannot be obtained.
As shown in FIG. 1, the top-emission type LED employing the nitride-based semiconductor includes a p-ohmic contact layer that can be obtained by stacking thin nickel (Ni) or a thick transparent conducting layer, such as, gold (Au) or indium tin oxide (ITO), on a p-nitride cladding layer and then annealing the p-nitride cladding layer in a oxygen (O2) atmosphere or in a nitrogen (N2) atmosphere. In particular, when the ohmic contact layer including semi-transparent nickel-gold (Ni—Au) and having a low non-contact resistance value of about 10−3 cm2 to 10−4 cm2 is subject to the annealing process at the temperature of about 500° C., nickel oxide (NiO), which is p-semiconductor oxide, is distributed in the form of an island on the interfacial surface between the p-nitride-based cladding layer and the nickel-gold ohmic contact layer. In addition, gold (Au) particles having superior conductivity are embedded into the island-shaped nickel oxide (NiO), thereby forming a micro structure. Such a micro structure may reduce the height and width of the Schottky barrier formed between the p-nitride cladding layer and the nickel-gold ohmic contact layer, provide hole carriers in the n-nitride cladding layer, and distribute gold (Au) having superior conductivity, thereby achieving superior current spreading performance. However, since the nitride-based top emission type LED employing the p-ohmic contact layer consisting of nickel-gold (Ni—Au) includes gold (Au) that reduces the light transmittance, the nitride-based top emission type LED represents a low EQE (external quantum efficiency), so the nitride-based top emission type LED is not suitable for the next-generation LED requiring a high capacity, large area and high brightness.
There exists another method of providing a p-ohmic contact layer without using the semi-transparent Ni—Au. According to this method, the p-ohmic contact layer is obtained by directly depositing a transparent conducting oxide layer including a thick transparent conducting material, such as indium (In), tin (Sn) or zinc (Zn) which is known in the art as a material for a high transparent ohmic contact electrode, and a transparent conducting nitride layer including transition metal, such as titanium (Ti) or tantalum (Ta), on a p-nitride-based cladding layer. However, although the ohmic electrode fabricated through the above method can improve the light transmittance, the interfacial characteristic between the ohmic electrode and the p-nitride-based cladding layer is deteriorated, so the ohmic electrode is not suitable for the top emission type nitride-based LED.
Various documents (for example, IEEE PTL, Y. C. Lin, etc. Vol. 14, 1668 and IEEE PTL, Shyi-Ming Pan, etc. Vol. 15, 646) describe a nitride-based top emission type LED having superior electrical and thermal stability and representing the great EQE by employing a p-ohmic contact layer, which is obtained by combining a transparent conducting oxide layer having superior electrical conductivity with a metal, such as nickel (Ni) or ruthenium (Ru), without using a noble metal, such as gold (Au) or a platinum (Pt) in such a manner that the p-ohmic contact layer has light transmittance higher than that of the conventional p-ohmic contact layer of a nickel-gold (Ni—Au) electrode.
Also described is a nitride-based top emission type LED which employs an indium tin oxide (ITO) transparent layer as a p-ohmic contact layer and represents an output power higher than that of a conventional LED employing the conventional nickel-gold (Ni—Au) ohmic electrode. However, although the p-ohmic contact layer employing the ITO transparent layer can maximize the EQE of the LED, a relatively large amount of heat is generated when the nitride-based LED is operated because the p-ohmic contact layer has a relatively high non-contact ohmic resistance value, such the above p-ohmic contact layer is not suitable for the nitride-based LED having the large area, high capacity, and high brightness.
In order to improve the electrical characteristics of the LED, which may be degraded due to the ITO electrode, LumiLeds Lighting Company (U.S.) has developed an LED having higher light transmittance and superior electrical characteristics by combining indium tin oxide (ITO) with thin nickel-gold (Ni—Au) or thin nickel-silver (Ni—Ag) (U.S. Pat. No. 6,287,947 issued to Michael J. Ludowise etc.). However, the LED described in the above patent requires a complicated process to form a p-ohmic contact layer and employs gold (Au) or silver (Ag), so this LED is not suitable for the nitride-based LED having the high capacity, large area and high brightness.
A new nitride-based top emission type LED provided with a high-quality p-ohmic contact layer has been developed. According to the above nitride-based top emission type LED, new transparent nano particles having sizes of 100 nanometers (nm) or less are provided onto an interfacial surface between a p-nitride-based cladding layer and a transparent conducing oxide electrode, such as an ITO electrode, so as to improve the electrical characteristics.
Technologies related to the fabrication of the nitride-based LED have also been developed. For instance, in order to directly use a highly transparent conducting layer (ITO layer or TiN layer) as a p-ohmic contact layer, as shown in FIG. 2, the transparent conducting layer (ITO layer or TiN layer) is deposited onto a super lattice structure including +-InGaN/n-GaN, n+-GaN/n-InGaN, or n+-InGaN/n-InGaN after repeatedly growing the super lattice structure on an upper surface of a p-nitride-based cladding layer. Then, a high-quality n-ohmic contact is formed through an annealing process and a tunneling junction process is performed, thereby obtaining the nitride-based LED having the high quality.
FIG. 2 is a cross-sectional view showing another conventional nitride-based top emission type light emitting device available from various companies and research institutes of Taiwan, Japan and U.S.
A nitride-based buffer layer 220, an n-nitride-based cladding layer 230, a multiple quantum well nitride-based active layer 240, a p-nitride-based cladding layer 250, and a p-ohmic contact layer 160 are sequentially formed on a substrate 210. Reference numerals 270 and 280 represent a p-type electrode pad and an n-type electrode pad, respectively.
The conventional nitride-based top emission type light emitting device of FIG. 2 includes the p-ohmic contact layer 260 consisting of a transparent conducting oxide layer, such as an indium tin oxide (ITO) layer 260b, and a super lattice layer 260a, such as an In—Ga—N layer.
A thin single crystal dual layer consisting of In(Al)GaN and Al(In)GaN is repeatedly deposited on the p-nitride-based cladding layer 250 before the ITO layer 260b, which serves as a current spread layer, thereby forming the super lattice layer 260a. 
According to another technology, as shown in FIG. 3, a semiconductor layer with a narrow band gap, such as p+-AlInGaN, GaN, (Al)GaAs, GaP, or AlGaP having a high hole density of 1018/cm3 or more, is grown from an upper surface of a p-nitride-based cladding layer and a transparent conducting layer is deposited onto the semiconductor layer. In this state, an annealing process is performed such that a p-ohmic contact layer can be formed through the tunneling effect.
FIG. 3 is a cross-sectional view showing another conventional nitride-based top emission type light emitting device, such as is available from Epistar Company of Taiwan.
A nitride-based buffer layer 320, an n-nitride-based cladding layer 330, a multiple quantum well nitride-based active layer 340, a p-nitride-based cladding layer 350, and a p-ohmic contact layer 360 are sequentially formed on a substrate 310. Reference numerals 370 and 380 represent a p-type electrode pad and an n-type electrode pad, respectively.
Referring to FIG. 3, a second p+ cladding layer 360a having a higher hole density is deposited on the p-nitride-based cladding layer 350 before a transparent conducting oxide layer 360b is deposited on the p-nitride-based cladding layer 350. Instead of an ohmic contact electrode including semi-transparent nickel-gold (Ni—Au), a transparent conducting oxide layer 360b, such as an ITO layer served as a current spread layer, is used as a p-ohmic contact layer 360. The second p+ cladding layer 360a having the higher density of 5×1018/cm3 includes a single crystal epitaxial layer consisting of Al—In—Ga—N, Al—In—Ga—As, or Al—In—Ga—P.
However, the above technologies are not suitable for the nitride-based LED requiring the high capacity, large area and high brightness due to the following basic difficulties related to materials and technologies.
First, since the LED having the high capacity, large area and high brightness is fabricated through the tunneling junction process by using the super lattice structure including an epitaxial nitride-based semiconductor, the LED generates a great amount of heat during the operation thereof, thereby causing serious voltage drop and shortening the life span of the LED.
Second, since a single crystal p+ cladding layer having a crystal size smaller than that of the p-nitride-based cladding layer is provided onto the p-nitride-based cladding layer so as to form the p-ohmic contact layer, light having a short wavelength generated from the nitride-based active layer is absorbed through the p-ohmic contact electrode, so that the EQE is lowered.
In other words, since the above technologies form the p-ohmic contact layer by inducing the tunneling effect after interposing the single crystal super lattice structure having a thin thickness or a second p+ cladding layer having a narrow band gas between the p-nitride-based cladding layer and the transparent p-ohmic contact layer, the nitride-based top emission type LED fabricated through the above conventional technologies represents the low EQE while being operated at a higher operational voltage. Thus, the above conventional technologies are not suitable for the next-generation nitride-based top emission type LED having the high capacity, large area, and high brightness.